Patent GB-1494510-A (GB)
Title: Tape playback apparatus including timing error correcting means
IPC: A61B5/336
Filed: 1974-12-24
Abstract: 1494510 Correcting timing errors in tape playback AMERICAN OPTICAL CORP 24 Dec 1974 [26 Dec 1973] 55776/74 Heading G5R In a tape playback apparatus arranged to derive a time-error compensated signal from a tape 60 recorded with an original analogue signal and a clock signal and having a nominal recording speed of 1 inch/sec. or less, the separate analogue and clock signals are derived from the tape during playback, control means responsive to the derived clock signals controls the tape playback speed so that changes therein which are slower than the response interval of the control means are compensated for so that the average tape speed averaged over an interval at least as long as the response interval of the control means remains constant, and the control means generates a corrected clock signal of constant frequency equal to the average frequency of the off-tape clock signal averaged over an interval at least as long as the response interval of the control means, sampling means 20 taking samples of the off-tape analogue signal, the samples being taken in sync with the off-tape clock signals, and means 30 controlled by the corrected generated clock signal operating at equally spaced intervals on the successive samples taken by the sampler 20 to reconstitute from said samples the original analogue signal or successive digital representations thereof. In apparatus for playback of a magnetic tape recorded at 0À08 inches/sec, with ECG signals, over a 24 hour period for example, the ECG signals having been FM or directly recorded in analogue form on the same or a separate track as a 240 Hz square wave clock signal, the offtape analogue and clock signals pass through low-pass 16 and high-pass filters 18 respectively and an input sample and hold circuit 20, which may comprise a plurality of successively operated sample and hold circuits, samples the analogue signal under the control of a clock signal which occurs at an average rate of 480 Hz and is derived from the 240 Hz signal by differentiator and rectifier 24. A phase-locked loop circuit 26, including a synchronous demodulator 60, produces at the output of a filter 62, 64 a slow-changing D.C. signal representing the average rate of the off-tape clock signal on line 22. The output of filter 62, 64 controls a voltage controlled oscillator 66 which produces on line 34 the corrected clock signal having a frequency of 240 Hz equal to the average frequency of the off-tape clock signal. A differentiator and rectifier 36 gives a 480 Hz signal from the signal on line 34 and controls an output sample and hold circuit 30 receiving an input from circuit 20 so that the output of filter 32 is the reconstituted original analogue signal. The clock signal on line 34 is in quadrature with that on line 22 so that operations of circuit 30 alternate with those of circuit 20. The output of filter 62, 64 on line 38 and the output of the tacho, of a motor-tachometer 52 driving tape capstan 54 are subtracted in an adder 40 from an input thereto derived from a source 42 by resistor 46 which is adjustable to set the tape speed, the output of adder 40 controlling the speed of motor 52 so that slow changes in tape speed are corrected and the off-tape clock signals on line 14 have an average rate equal to that used in recording. For improved speed correction, the clock signal on line 34 is compared with the output of a stabilized oscillator by passing both outputs to an up-down counter, each oscillator pulse increasing the count and each step in the waveform on line 34 decreasing the count, and the D.C. voltage output of the counter, proportional to the total count therein, is passed to adder 40 instead of the signal on line 38. In a modification, circuits 20 and 30 are replaced by an analogue-to-digital converter followed by a first-in, first-out buffer memory, the clock pulses from circuit 24 controlling successive conversions to digital form of the off-tape analogue signal on line 12, and the corrected clock pulses from circuit 36 controlling the storage of the digitized samples in the buffer memory and the read out therefrom. The apparatus may have a tape drive as described in Specification 1,473,168 and a head mounting as described in Specification 1,476,755.